Adult sea scallops reproduce by freely releasing gametes into the water column where larvae swim and develop for a period of about 50 days. Normally, larvae ready to settle find a likely spot on the bottom and begin to develop into the juvenile form. This newly settled stage is called “spat” which can attain size of 1 cm within a few weeks. It has been shown in countries such as Japan that commercially important sea scallop stocks may be greatly enhanced and managed through the practice of spat collection. Meshed bags or collectors are placed in regions where larvae are naturally concentrated, the larvae settle on the collectors and quickly develop into spat. The spat are then relayed into protected grow-out areas where fishing pressure is eliminated. After 1 to 2 years, the adult scallops are harvested using normal bottom trawling techniques.
Knowledge of the population dynamics of bivalves such as clams, oysters, and scallops is required to effectively management these commercially important shellfish resources. A major stumbling block to understanding bivalve population dynamics has been identification of the planktonic, and very small (order 100 microns) larvae. For years researches have searched for distinguishing, species-specific features for identifying larvae. Although scanning electron microscopy (SEM) is regarded as the most reliable approach for feature identification, the technique is very laborious requiring days to process just a few larvae. The optical approach to larval identification described here lends itself to non-destructive, rapid, flow-through optical systems where hundreds of larvae could be identified in seconds. When put into use on research vessels, this approach could greatly enhance our ability to map abundance and distribution of bivalve larvae in the world ocean and allow researchers to make rapid predictions of where the ocean currents will carry the larvae before settlement on the bottom. Resource managers require this kind of information before attempting to establish effective management plans.
Many bivalve mollusks produce planktonic larvae which progress through a series of developmental stages before settling to the bottom and taking up a benthic existence. All bivalve mollusks have larvae which progress through a series of developmental stages: fertilization of the egg, gastrulation, trochophore stage, prodissochonch I stage, prodissochonch II stage, and dissochonch stage. Mineralization of the prodissochonch I shell begins about 20 h after fertilization in an area of the trochophore larva called the shell field invagination (SFI), which consists of a proteinacous organic matrix. Optical orientation of the bi-refringent aragonitic crystals, which nucleate and grow on the organic matrix, is controlled by protein organization within the organic matrix. Under polarized light, the prodissochonch I shell exhibits a dark cross of full light extinction where the optic axes of the two polarizing filters are normal to one another demonstrating that crystal orientation is arranged radially around the SFI. The mode of shell deposition makes a transition between prodissoconch I/II formation as crystals are added to the ventral margins of the shell with an orientation relative to the previous layer. This process continues for about 30 days until the prodissochonch II shell is fully formed and the larva is ready to metamorphose into the dissoconch stage. The prodissoconch I/II boundary is clearly viewed under polarized light extending radially around the shell at a distance of about 100 microns from the SFI.
Since bivalve larvae are typically about 100 um in size, larvae species appear nearly identical upon conventional polarized light microscopy examination. A recent review of current identification techniques has been provided by Garland and Zimmer (E. D. Garland and C. A. Zimmer, Mar. Ecol. Prog. Ser., 225:299-310 (Jan. 11, 2002). Attempts to identify bivalve larvae based on their morphology such as shell shape (Chanley and Andrews, 1971; Loosanoffet al. 1966; Miyazaki, 1962; Rees, 1950; Stafford, 1912) or denticular structures (Le Pennec, 1980; Lutz et al. 1982) has been shown to be extremely, difficult, time consuming, and inaccurate. Use of the scanning electron microscope to identify denticular structures improves identification accuracy, but is extremely time consuming requiring hours to days for sample preparation and observation (Fuller et al. 1993; Lutz and Jablonski, 1979). More recently, a limited number of attempts to identify bivalve larvae to species have been made using molecular techniques (Bell and Grassle, 1997; 1998; Coffroth and Mulawka, 1995; Demers et al. 1993; Hu et al. 1992), but these are highly inaccurate, time consuming, and completely destructive to the individual. Thus, despite years of research we do not have the ability to accurately, repeatedly, and quickly identify scallop larvae from the plankton.
What is currently needed is a system and method for rapid identification of scallop and other bivalve larvae in the field using instrumentation on ships of opportunity where data can be immediately telemetered to shore, assimilated into physical models, and results made available on a daily basis to commercial fishermen.